1. Field of the Invention
The present invention relates to a method and apparatus for metal machining using a cutting tool die that pivots as it cuts a rotating metal work piece. By pivoting the cutting tool, the orientation of the metal surface being cut in the workpiece may be varied along the cutting path. A particular application of the invention is for Swiss lathe, e.g., a Swiss-type screw cutting machine, that cuts screw threads in screw blank rods, such that the threads have a variable cant along the length of the screw. The invention is not limited to the manufacture of screws, and has broad applicability to machining operations in which a variably canted surface in a workpiece is desired.
2. Description of the Prior Art
Prior Swiss-type lathes are not capable of cutting a helical groove in a metal surface, where the pitch of one or both of the groove walls is continuously varied during the cutting process. Swiss lathes of the type that preceded the present invention are used to machine metal devices from cylindrical rod stock. In a Swiss-type lathe, the chuck holds the cylindrical rod stock and moves the rod stock in a reciprocal and rotating manner. The chuck is generally behind the work area for the Swiss lathe, and near the work area the lathe has a collar to provide a steady-rest for the work area. The cutting tools of a Swiss-type lathe do not move longitudinally, but instead the lathe reciprocally moves the work piece. These Swiss lathes have a rotating chuck that turns the rod stock during the cutting process. The chuck also moves the rod stock laterally forward during the cutting process, and retracts the rod stock when the cutting tool is removed. As the lathe rotates the rod stock and/or moving the stock laterally, a cutting die mounted on the machine is brought into engagement with the rod stock to make a desired cut in the metal. A steady rest collar attached to the lathe supports the rod stock at a region of the stock proximate to the cutting operation.
The cutting tools associated with prior machines were primarily fixed dies, e.g., stick tools, that were mounted in tool brackets. These fixed dies were not moved while they are cutting into the rotating rod stock. With these fixed dies, the rod stock moved during the cutting operation, and the cutting die remained stationary. Certain specialty cutting tools have also been used with Swiss lathes, such as rotating drill bits and groove cutters, e.g. signal point carbide cutting tools. With drills, the rod stock was held stationary and the drill rotates to cuts a hole in the rod stock that is coaxial to the rotation of the drill bit. A groove cutter is similar to a drill bit, in that rapidly rotates 360.degree. about the axis of the cutter, but instead of forming a cylindrical hole as does a drill bit, the groove cutter forms a groove in the cutting stock. These prior art cutting dies, bits and cutters are not capable of making a cut in a rotating rod stock, where the pitch of the cut varies along the length of the cut.
There has been a long-felt need in the prior art for a metal cutting tool that can cut a helical groove in a rod stock, where the walls of the groove are variably canted. In 1990, a novel screw was developed having threads with variably canted surfaces. This novel screw is described in U.S. Pat. No. 5,492,442, and other related patents. When the screw was first developed, it could be machined by hand or by casting. Later, one of the inventors of the present invention, Mr. Andrew Pakos, developed a pivotable guide plate to hold a conventional lathe cutting tool to cut screws having variably canted threads. See U.S. Pat. No. 5,221,234. The guide plate has a armature that is pivoted by a cam mounted on the lathe. The guide plate is limited to conventional lathe applications and is not suitable for a Swiss-type lathes, such as used with the present invention.